A nitride semiconductor such as GaN (gallium nitride) has high electron mobility, and preferable for a power system transistor such as a high electron mobility transistor (HEMT), for example.
The high electron mobility transistor is prepared by using a nitride semiconductor substrate in which a nitride semiconductor layer is formed on a different kind of substrate, for example. The nitride semiconductor substrate has structure in which an initial layer constituted of AlN (aluminum nitride), SiC (silicon carbide) or SiN (silicon nitride) is formed on one principal plane of a single crystal substrate constituted of Si (silicon), SiC or sapphire, and a nitride semiconductor layer of GaN or the like is formed on the initial layer.